I dont know why but it is known that seasons can affect such things
You have a seasonal anxiety disorder. There could be many causes for it. My friend has it in the summer because she is very money conscious, does not like to spend money, and the summertime is when vacations are taken and money is spent.
You could approach your doctor about this. In the mean time you can do some breathing exercises or meditation.
Breathe in through your nose while your hands are on your stomach so you can feel it rise, hold your breath for a count of 5 and count it out in your head, then let it out through your mouth. Do this as many times as necessary to slow your racing heart.